1. Field of the Invention
This invention relates to a reproducing circuit for a video tape recorder and more particularly to a differential gain control circuit installed in the reproducing circuit.
2. Description of the Prior Art
Conventional circuits for prior art VTR video tape recorders are known. In the recording circuit of prior art VTR devices a color video signal of, for example, a NTSC system is fed to a low pass filter and to a band pass filter for separating the luminance and chrominance signals (the color subcarrier frequency being 3.58 MHz). The separated luminance signal is fed through a pre-emphasis circuit to an FM modulating circuit and the chrominance signal is fed to a frequency converting circuit for converting it to a frequency from 3.58 MHz to 688 KHz, for example. The FM modulated luminance signal and the frequency converted chrominance signal are then supplied to an adding circuit and the thus combined FM luminance signal and the chrominance signal are supplied through a recording amplifier to a pair of rotary magnetic heads. These heads are mounted on a rotary disc or drum spaced apart from each other by an angular distance of 180.degree.. These heads form skewed record tracks on a magnetic tape as one record track for each field.
FIG. 1 illustrates a prior art reproducing circuit for the prior art video tape recorder. In FIG. 1, a pair of rotary magnetic heads Ha and Hb mounted on a disc DS 180.degree. from each other are rotated by a motor Mr which has its output shaft AX connected to the disc DS. A pair of magnets MG are mounted on the axis AX 180.degree. relative to each other and a magnetic pick-up H.sub.p is mounted so as to detect the magnetic field produced by the magnets MG and generates a 30 Hz signal in case of the NTSC color signal. The outputs of the pair of rotary magnetic heads Ha and Hb are respectively supplied to amplifiers 1a and 1b which have their outputs connected to terminals of a change-over switch SW1 and which changes over at every field. The pulse signal from the rotary position detecting device PG is supplied to a switching control signal generating circuit 13 which produces a switching pulse to the change-over switch SW1 so as to change the switch-over at every field.
The output of the change-over switch SW1 is supplied to a high pass filter 2 for deriving the FM luminance signal and to a low pass filter 3 for deriving the frequency converter chrominance signal. The output of filter 2 is applied to an FM demodulating circuit 4 which demodulates it and the demodulated luminance signal is supplied to a de-emphasis circuit 5 which furnishes its output to an adding circuit 6.
The frequency converted chrominance signal is supplied from the filter 3 to a gain control 8 which supplies its output to a frequency reconverting circuit 9 which supplies its output to the adder 6. The chrominance signal, (the color subcarrier frequency of which is 3.58 MHz) from the converter 9 is applied to the adding circuit 6 where the chrominance and luminance signals are combined after being added and applied to the output terminal 7.
The chrominance signal from the frequency reconverting circuit 9 is also fed to a burst gate circuit 10 where the color burst signal of 3.58 MHz is extracted. The burst signal thus extracted is supplied to an automatic phase control circuit 11 which receives a reference signal from a reference oscillator circuit 12 and supplies an input to the converter 9. An output carrier 4.27 MHz generated in the automatic phase control circuit 11 is supplied to the frequency reconverting circuit 9.
An automatic color control circuit 20 provides an input to the variable gain control circuit 8 which is in the signal transmission path for the chrominance signal. The automatic color control generating circuit 21 includes the burst gate generator 10 which supplies an output to an AM detecting circuit 14 which detects the envelope of the applied signal supplied with the burst signal from the burst gate circuit 10. The AM detecting circuit 14 includes a switchable time constant circuit formed of capacitors 15a and 15b which have first sides connected to ground and other sides connected to contacts of switch SW2 which is connected to the output of the detector 14. The switch SW2 is controlled by the field burst from the switching control signal generator 13 so as to change the level of the chrominance signal as a function of the position of the rotary magnetic heads Ha and Hb. The output of the detector 14 is connected to the gain control terminal of the gain control circuit 8.
A differential gain DG is induced in the electromagnetic converting system because the low frequency converted chrominance signal is mixed with the FM modulated luminance signal and is thereafter recorded and/or reproduced. In the example illustrated in FIG. 1, the FM modulated luminance signal becomes an AC bias signal for the frequency converted chrominance signal during recording so that the differential gain DG depends upon the variation of the luminance signal. Thus, the differential gain DG cannot be corrected by a conventional ACC circuit. This differential gain will be greater deteriorated each time that dubbing procedures are repeated.